Most cases of infectious mononucleosis (IM) are caused by the Epstein-Barr virus (EBV); however a smaller portion results from infection with cytomegalovirus (CMV) adenovirus, rubella and toxoplasma. One of the pathogenic lesions common to the syndrome may be an inappropriately regulated induction of cytotoxic cells. The atypical lymphocytes of EBV-IM are primarily of T-cell origin and are cytotoxic for lymphoid cell lines (LCL) which carry the virus. Part of the effector response may be specific for EBV, but we find similarly cytotoxic cells in patients with EMV infections. We have therefore proposed that viral antigens may stimulate lymphocytes to develop into cytotoxic cultures are separated according to size by velocity sedimentation and the characteristics and functions of isolated populations determined. Comparison is made with cells from the blood of IM patients which have been similarly separated. The possible relevance of such blast cells to control of potentially malignant proliferation is being examined.